In the production of paper and paperboard, it is very often an objective to produce as smooth a finish as possible on at least one surface of the web, but at the same time to minimize compaction or densification of the web as a whole, especially in the case of paperboard.
Conventional practice has employed a variety of techniques and equipment in an effort to accomplish this result, including various forms of calendering, machine glazing by means of an M.G. cylinder or Yankee dryer, the use of a nip roll on a dryer roll in the dryer section, and a breaker stack. All of these techniques and equipment have some disadvantages.
More specifically, the nip pressures are so high in a calender stack, due to the weight and number of its component rolls, that substantial densification of the web is unavoidable. Machine glazing is more satisfactory from the standpoint of less densification of the web as a whole, but the cylinders used therefor are large and expensive, e.g. 15 to 20 feet in diameter, they are difficult to operate, and they require particular operating limitations related to the drying of the web which render their use impractical or uneconomic.
It is accordingly a primary object of the present invention to provide improved and simple methods and apparatus for the treatment of webs of paper and paperboard which will produce a satisfactorily smooth surface on one or both sides of the web with less densification than can be achieved by prior practice.